The NOAA Space Weather Scales translate solar activity into hurricane-style categories: G1–G5 for geomagnetic storms, S1–S5 for solar radiation storms, R1–R5 for radio blackouts. Each pairs a measured threshold with plain-language effects, from minor (1) to extreme (5).
Understanding NOAA Scales
The three hazards travel at three speeds
A single big eruption can raise all three flags — on staggered clocks. The flare's X-rays arrive with the light, eight minutes after the event: R-scale effects are effectively instantaneous, dayside HF fading while forecasters are still typing. The fastest protons follow within tens of minutes to hours, switching on the S scale, hazarding polar aviation and satellite electronics — and, for crewed flight, starting the sheltering clock. The CME itself lumbers behind at one to three days, and only on arrival — if its field turns southward — does the G scale light up with the geomagnetic storm proper. This choreography is why a major event unfolds as a sequence of separate warnings, and why the G-scale forecast carries the most uncertainty: everything depends on magnetic geometry that cannot be measured until the ejecta reaches the upstream monitors an hour out.
Reading the scales like an operator
| Level | G (geomagnetic) | S (radiation) | R (radio) |
|---|---|---|---|
| 1 — Minor | Weak grid fluctuations; polar aurora | Negligible biological risk | Brief HF degradation, dayside |
| 3 — Strong | Voltage corrections needed; aurora to mid-latitudes; drag up | Polar flights rerouted; satellite upsets likely | Wide-area HF loss ~1 hour |
| 5 — Extreme | Grid collapse possible; satellite ops disrupted; aurora near tropics | Astronaut hazard; electronics damage | Dayside HF blackout for hours |